The present invention relates to a carbonless copying paper having a coated layer containing microcapsules. More particularly, it relates to a carbonless copying paper excellent in resistance to smudging and in applicability of coating color. Furthermore, it relates to a carbonless copying paper which is excellent in feel and printability.
A carbonless copying paper is produced by coating a color developer and microcapsules containing a normally colorless leuco dye (color former) on a support. The color developer and the microcapsules are coated on a support separately which is used as a upper sheet and a lower sheet respectively; or, they are coated on front and back sides of a support respectively, which is used as a middle sheet. Furthermore, a self-contained copying paper, in which the color developer and the microcapsules are coated on the same side of a support in one or two layers, is also used. A plurality of these sheets which are suitably combined are applied with physical pressure by writing, typing, impact printer, and the like to rupture the microcapsules and release the leuco dye enclosed in the capsules, which contacts with the color developer to cause formation of color to obtain a recorded image.
In general, carbonless copying papers are required to fulfil the following requirements as important quality criteria.
1. Clear images of high density can be obtained in a short time and do not fade with the lapse of time.
2. Smudging, which is undesirable color formation, does not occur. That is, a carbonless copying paper must be resistant to such color formation during its production steps, cutting, printing, or storaging and handling in the form of sheets or rolls.
Since the above requirements 1 and 2 are contrary to each other, it is generally difficult to satisfy both of them and balance them. In general, a coating composition containing microcapsules comprises the microcapsules, a stilt agent, a binder and other additives, and by adjusting each of those components as exemplified in the following (a), (b) and (c), aforesaid conflicting requirements can be brought into a better balance, and a lot of proposals have been made in this regards:
(a) conditions for production of microcapsules; PA1 (b) selection of kind and amount of stilt agent; and PA1 (c) selection of kind and amount of binder. PA1 Dynamic smudging due to color formation caused by rubbing. PA1 Smudge due to color formation under a static pressure applied to the paper in the form of sheets or rolls during storage, transportation, printing or other processing steps such as Guillotine sheeting. PA1 Smudge due to color formation caused by rupture of microcapsules when the paper is stored for a long time under high temperature and humid conditions. PA1 Another object of the present invention is to additionally improve the following characteristics C. PA1 Feel (namely, the smooth coated surface good in hand feeling.) PA1 Printability (good ink transfer on a press.) PA1 Coating processibility on a contact coating system (namely, there occurs no rupture of microcapsules during coating.)
With reference to the method for production of microcapsules enclosing color former of (a), many methods have been known and representative ones are the following.
A coacervation method utilizing a poly-ion complex of gelatin and gum arabic.
An interfacial polymerization method, according to which a hydrophobic solution to be encapsulated is dispersed in a hydrophilic medium to allow formation of a water-insoluble film at the interface of said dispersed hydrophobic solution.
An in-situ polymerization method according to which a film forming resin precondensate such as melamine-formaline resin, urea-formalin resin, or the like is added to an aqueous dispersion in which a hydrophobic solution to be encapsulated has been dispersed and emulsified to allow polymerization of the resin precondensate on the surface of the emulsified solution particles.
Recently, synthetic resin microcapsules, especially those which are produced by the in-situ method, come to be used increasingly because the starting materials can be supplied inexpensively and stably, a microcapsule emulsion of high concentration can be obtained and the production process is simple.
As a means to attain a better balance of said requirements 1 and 2, aggregation of anionic microcapsules by use of an amphoteric polyelectrolyte such as gelatin, and hardening the aggregated capsule wall by use of formaldehyde or the like has been proposed. (Japanese Patent Kokai No. Sho 60-166026).
With reference to the stilt agent of the above (b), there have been used, in an amount of 10-400 parts by weight per 100 parts by solid weight of microcapsules of glass beads disclosed in U.S. Pat. No. 2,711,375, pulverized cellulosic fibers (cellulose flocks) disclosed in U.S. Pat. No. 2,711,375, and ungelatinized starch particles (wheat starch, potato starch, pea flower starch, and the like) disclosed in Japanese Patent Kokoku Nos. Sho 47-1178 and 48-33204. In general, these stilt agents are inert particles (generally having a diameter of 5-50 .mu.m) somewhat larger than the microcapsule particle.
The stilt agent is very important for protection of microcapsules, but when a coating composition containing it is applied by a contact coating system such as a blade coater (flexible blade coater, trailing blade coater, fountain blade coater and the like) and a rod bar coater, the agent tends to be scraped off, and sometimes fails to be retained sufficiently. Besides, use of it results in a conspicuously ragged coated surface affecting harmfully on feel and printability (especially transferability of ink) of the microcapsules coated paper. Recently, customers have come to demand not only functional performance of carbonless copying paper but also its beautifulness including feel and texture of the paper. Moreover, market of the paper is ever becoming diversified and needs to print on microcapsule coated surface of the paper are increasing. To comply with these needs, coating compositions containing no stilt agent have been proposed. (Japanese Patent Kokai Nos. 61-151271, 61-192586, 61-241185, and 61-241186).
With reference to the above (c), selection of kind and amount of binders, use of various latexes has been proposed. Use of them is also disclosed in the above-mentioned coating compositions containing no stilt agent. As compared to other binders known in the art (polyvinyl alcohol, starch, and the like), a latex is thought to have the merit by itself of higher cushioning action. On the other hand, a latex tends to penetrate into the support as it comprises fine emulsified particles, so that merit fails to be brought up sufficiently in many cases.
To suppress penetration of a latex, it has been proposed to employ totally or partially an alkali-soluble emulsion type latex which thickens the coating composition (Japanese Patent Kokai Nos. Sho 64-49678 and Hei 1-234289, 234290, 1-288480, and 2-3367).
However, the above various proposals have not yet resulted in satisfactory products.